(1) Field of the Invention
The present invention is directed to a dual stacked beam radar having combined interpulse period sequencing and an antenna having two groups of multiple beams stacked in elevation. Thus, half the usual number of receivers and signal processors are employed and a smoother velocity response than prior art devices is obtained by interleaving the interpulse periods.
(2) Description of Related Art
Two types of stacked beam radars have been employed in the past. The most common type is a simultaneous stacked beam configuration which employs N receivers (N being an integer) for listening simultaneously to N beams. The transmit pattern is shaped to obtain the desired power at each elevation direction. Receivers listen to echoes from each transmission in an individual receive beam. Because long range reception demands maximum power at low elevations, surface clutter is strongly illuminated and the receive beam sidelobes rarely provide adequate suppression of surface clutter interference. Therefore, each beam must include Doppler filtering to suppress the surface clutter interference.
Doppler filters add to the cost of each receiver and also introduce a sensitivity loss of several decibels (dB) averaged over all target radial velocities. At dim speeds (the radial speed at which there occurs a significant loss of sensitivity with respect to the average of all radial velocities), the loss can be many dB. Both factors require extra power to be transmitted at high elevation angles to compensate for the loss. The simultaneous stacked beam radar requires maximum transmitter cost as well as maximum receiver cost.
The second type of stacked beam radar is a sequential stacked beam configuration. This employs an array antenna which steers its beams in elevation by controlling either frequency or phase shifters, or both. In radar of this type, all beams have virtually the same bandwidth. Narrow beams are required at low elevations for long range detection and height accuracy. This results in a larger number of beams being required in order for the system to operate. To avoid a corresponding increase in the number of receivers, the elevation coverage is divided into multiple zones and is inspected sequentially. More pulses are transmitted in the lower beams where clutter interference is most severe and where power requirements are a maximum. Clutter in the higher beams is dictated by two-way sidelobes or frequency discrimination so that the high elevation beams use fewer pulses and do not require Doppler filtering. The absence of loss associated with Doppler filtering in the higher beams is an advantage of this configuration, but is achieved by stealing valuable dwell time (the time for the antenna to scan in the azimuth over a 6 dB two-way beamwidth) from the low elevation beams. Fewer interpulse periods are available to smooth the velocity response of the Doppler filters required for these beams.
The interpulse period is the time between transmitted pulses. If a fixed interpulse period is maintained with Doppler filtering, there are complete blind speeds at Doppler frequencies corresponding to integer multiples of the pulse repetition frequency (i.e., the inverse of the interpulse period). It is customary, therefore, to vary the interpulse period during the time the beam is on the target to fill in the blind speeds.
A smooth velocity response requires the use of a large number of different interpulse periods during each dwell time, the maximum interpulse period approaching twice the minimum interpulse period.
The present invention combines both techniques and overcomes the problems associated therewith.